REGENXBIO (RGNX) 10-Q 7 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 01, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	RGNX
Entity Registrant Name	REGENXBIO Inc.
Entity Central Index Key	0001590877
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-37553
Entity Tax Identification Number	47-1851754
Entity Address, Address Line One	9600 Blackwell Road
Entity Address, Address Line Two	Suite 210
Entity Address, City or Town	Rockville
Entity Address, State or Province	MD
Entity Address, Postal Zip Code	20850
City Area Code	240
Local Phone Number	552-8181
Entity Common Stock, Shares Outstanding		37,241,119
Document Transition Report	false
Document Quarterly Report	true
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Entity Incorporation, State or Country Code	DE

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 69,411	$ 69,514
Marketable securities	209,846	226,696
Accounts receivable	44,522	38,148
Prepaid expenses	7,422	6,475
Other current assets	7,032	4,199
Total current assets	338,233	345,032
Marketable securities	77,361	103,785
Accounts receivable	4,373	4,155
Property and equipment, net	30,414	28,973
Operating lease right-of-use assets	9,375	10,078
Restricted cash	1,330	1,330
Other assets	2,850	4,555
Total assets	463,936	497,908
Current liabilities
Accounts payable	9,008	6,409
Accrued expenses and other current liabilities	20,327	24,846
Operating lease liabilities	2,454	2,421
Total current liabilities	31,789	33,676
Deferred revenue	3,333	3,333
Operating lease liabilities	7,990	8,874
Other liabilities	672	1,828
Total liabilities	43,784	47,711
Stockholders’ equity
Preferred stock; $0.0001 par value; 10,000 shares authorized, and no shares issued and outstanding at March 31, 2020 and December 31, 2019
Common stock; $0.0001 par value; 100,000 shares authorized at March 31, 2020 and December 31, 2019; 37,190 and 36,992 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	4	4
Additional paid-in capital	638,588	627,810
Accumulated other comprehensive income (loss)	(580)	205
Accumulated deficit	(217,860)	(177,822)
Total stockholders’ equity	420,152	450,197
Total liabilities and stockholders’ equity	$ 463,936	$ 497,908

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	37,190,000	36,992,000
Common stock, shares outstanding	37,190,000	36,992,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues
Revenues	$ 17,644	$ 884
Types of Revenue[Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating Expenses
Cost of revenues	$ 3,409	$ 29
Research and development	37,035	25,203
General and administrative	14,833	11,558
Other operating expenses	67
Total operating expenses	55,344	36,790
Loss from operations	(37,700)	(35,906)
Other Income (Loss)
Interest income from licensing	848	613
Investment income (loss)	(3,186)	2,995
Total other income (loss)	(2,338)	3,608
Loss before income taxes	(40,038)	(32,298)
Income Tax Benefit		70
Net loss	(40,038)	(32,228)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net	(785)	621
Total other comprehensive income (loss)	(785)	621
Comprehensive loss	$ (40,823)	$ (31,607)
Basic and diluted net loss per share	$ (1.08)	$ (0.89)
Weighted-average basic and diluted common shares outstanding	37,104	36,366

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid in capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2018	$ 508,848	$ 4	$ 592,580	$ (720)	$ (83,016)
Balances (Shares) at Dec. 31, 2018		36,120
Adoption of ASU | ASU 2016-02 (Topic 842) [Member]	(33)				(33)
Adoption of ASU | ASU 2018-02 [Member]				40	(40)
Exercise of stock options	3,762		3,762
Exercise of stock options, Shares		481
Issuance of common stock under employee stock purchase plan	365		365
Issuance of common stock under employee stock purchase plan, shares		10
Stock-based compensation expense	5,718		5,718
Unrealized gain (loss) on available-for-sale securities, net	621			621
Net income (loss)	(32,228)				(32,228)
Balances at Mar. 31, 2019	487,053	$ 4	602,425	(59)	(115,317)
Balances (Shares) at Mar. 31, 2019		36,611
Balances at Dec. 31, 2019	450,197	$ 4	627,810	205	(177,822)
Balances (Shares) at Dec. 31, 2019		36,992
Exercise of stock options	2,154		2,154
Exercise of stock options, Shares		181
Issuance of common stock under employee stock purchase plan	607		607
Issuance of common stock under employee stock purchase plan, shares		17
Stock-based compensation expense	8,017		8,017
Unrealized gain (loss) on available-for-sale securities, net	(785)			(785)
Net income (loss)	(40,038)				(40,038)
Balances at Mar. 31, 2020	$ 420,152	$ 4	$ 638,588	$ (580)	$ (217,860)
Balances (Shares) at Mar. 31, 2020		37,190

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net loss	$ (40,038)	$ (32,228)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	8,017	5,718
Net amortization of premiums and accretion of discounts on marketable debt securities	39	(368)
Depreciation and amortization	1,994	1,614
Net realized gains on sales and maturities of marketable securities	(7,085)
Imputed interest income from licensing	(848)	(613)
Unrealized losses on marketable equity securities	12,196
Other non-cash adjustments	25	327
Changes in operating assets and liabilities
Accounts receivable	(5,744)	591
Prepaid expenses	(947)	(790)
Other current assets	(2,833)	(295)
Operating lease right-of-use assets	703	573
Other assets	1,705	26
Accounts payable	3,063	316
Accrued expenses and other current liabilities	(3,813)	(2,932)
Operating lease liabilities	(851)	(592)
Other liabilities	(1,156)	(644)
Net cash used in operating activities	(35,573)	(29,297)
Cash flows from investing activities
Purchases of marketable debt securities	(30,692)	(79,249)
Maturities of marketable debt securities	60,907	87,165
Sales of marketable equity securities	7,124
Purchases of property and equipment	(4,630)	(2,455)
Net cash provided by investing activities	32,709	5,461
Cash flows from financing activities
Proceeds from exercise of stock options	2,154	3,762
Proceeds from issuance of common stock under employee stock purchase plan	607	365
Net cash provided by financing activities	2,761	4,127
Net decrease in cash and cash equivalents and restricted cash	(103)	(19,709)
Cash and cash equivalents and restricted cash
Beginning of period	70,844	76,614
End of period	$ 70,741	$ 56,905

Nature of Business

Nature of Business	3 Months Ended
	Mar. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) is a clinical-stage biotechnology company seeking to improve lives through the curative potential of gene therapy. The Company’s proprietary adeno-associated virus (AAV) gene delivery platform (NAV Technology Platform) consists of exclusive rights to over 100 novel AAV vectors, including AAV7, AAV8, AAV9 and AAVrh10. The NAV® Technology Platform is being applied by the Company, as well as by third-party licensees (NAV Technology Licensees), in the development of a broad pipeline of product candidates in multiple therapeutic areas. Additionally, the NAV Technology Platform is currently being applied in one commercially available product, Zolgensma®, which is marketed by a NAV Technology Licensee. The Company was formed in 2008 in the State of Delaware and is headquartered in Rockville, Maryland. Liquidity and Risks As of March 31, 2020, the Company had generated an accumulated deficit of $217.9 million since inception. As the Company has incurred cumulative losses since inception, transition to recurring profitability is dependent upon achieving a level of revenues adequate to support the Company’s cost structure, which depends heavily on the successful development, approval and commercialization of its product candidates. The Company may never achieve recurring profitability, and unless and until it does, the Company will continue to need to raise additional capital, to the extent possible. As of March 31, 2020, the Company had cash, cash equivalents and marketable securities of $356.6 million, which management believes is sufficient to fund operations for at least the next 12 months from the date these consolidated financial statements were issued. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, development by the Company or its competitors of technological innovations, risks of failure of clinical trials, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to transition from clinical manufacturing to the commercial production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). The interim unaudited consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements as of and for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, which was filed with the SEC on February 26, 2020. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which are normal and recurring in nature, necessary for fair financial statement presentation. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year, any other interim periods, or any future year or period. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements. Significant estimates are used in the following areas, among others: license and royalty revenue, stock-based compensation expense, accrued research and development expenses and other accrued liabilities, income taxes and the fair value of financial instruments. The Company is actively monitoring the impact of the novel coronavirus (COVID-19) pandemic on its business, results of operations and financial condition. The full extent to which COVID-19 will directly or indirectly impact the Company’s business, results of operations and financial condition in the future is unknown at this time and will depend on future developments that are highly unpredictable. The most significant estimates affecting the Company’s consolidated financial statements that may be impacted by the COVID-19 pandemic are related to the Company’s assessment of credit losses on accounts receivable, contract assets and available-for-sale debt securities. Reclassifications Certain amounts reported in prior periods have been reclassified to conform to current period financial statement presentation. These reclassifications are not material and have no effect on previously reported financial position, results of operations and cash flows. Restricted Cash Restricted cash includes money market mutual funds used to collateralize irrevocable letters of credit as required by the Company’s lease agreements. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): March 31, 2020 March 31, 2019 Cash and cash equivalents $ 69,411 $ 55,852 Restricted cash 1,330 1,053 Total cash and cash equivalents and restricted cash $ 70,741 $ 56,905 Accounts Receivable Accounts receivable primarily consist of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable include amounts invoiced to licensees as well as rights to consideration which have not yet been invoiced, including unbilled royalties, and for which payment is conditional solely upon the passage of time. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any accounts receivable from the licensee which are not contractually payable to the Company are charged off as a reduction of license revenue in the period of the termination. Accounts receivable which are not expected to be received by the Company within 12 months from the reporting date are stated net of a discount to present value and recorded as non-current assets on the consolidated balance sheets. The present value discount is recognized as a reduction of revenue in the period in which the accounts receivable are initially recorded and is accreted as interest income from licensing over the term of the receivables. Accounts receivable are stated net of an allowance for doubtful accounts, if deemed necessary based on the Company’s evaluation of collectability and potential credit losses. Management assesses the collectability of its accounts receivable using the specific identification of account balances, and considers the credit quality and financial condition of its significant customers, historical information regarding credit losses and the Company’s evaluation of current and expected future economic conditions. If necessary, an allowance for doubtful accounts is recorded against accounts receivable such that the carrying value of accounts receivable reflects the net amount expected to be collected. The Company did no t record an allowance for doubtful accounts as of March 31, 2020 and December 31, 2019 . Marketable Securities Marketable securities consist of available-for-sale debt securities and equity securities and are carried at fair value. Marketable debt securities with remaining maturity dates exceeding 12 months which are not intended to be sold prior to maturity for use in current operations are classified as non-current assets. Marketable equity securities are classified as current assets. Unrealized gains and losses on available-for-sale debt securities, net of any related tax effects, are excluded from results of operations and are included in other comprehensive income (loss) and reported as a separate component of stockholders’ equity until realized. The Company uses the aggregate portfolio approach to release the tax effects of unrealized gains and losses on available-for-sale debt securities in accumulated other comprehensive income (loss). Purchase premiums and discounts on marketable debt securities are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Unrealized gains and losses on marketable equity securities are included in results of operations as investment income (loss). Realized gains and losses from the sale or maturity of marketable securities are based on the specific identification method and are included in results of operations as investment income (loss). At each reporting date, the Company evaluates available-for-sale debt securities which have an amortized cost basis in excess of the fair value of the security to determine if the unrealized loss or any potential credit losses should be recognized in results of operations . If the Company does not have the intent and ability to hold the security until recovery of the unrealized loss, the difference between the fair value and amortized cost basis of the security is charged to results of operations resulting in a new amortized cost basis of the security. If the Company has the intent and ability to hold the security until recovery of the unrealized loss, the security is evaluated for potential credit losses. If a credit loss is deemed to exist, the credit loss is recognized in results of operations and an allowance for credit losses is recorded against the amortized cost basis of the security. In determining whether a credit loss exists related to impaired available-for-sale debt securities, the Company considers, among other factors, the extent of the unrealized loss relative to the amortized cost basis, the credit rating of the issuer and any recent changes thereto, current and expected future economic conditions, and any adverse events or other changes in circumstances that have occurred which may indicate a potential credit loss. The Company did not record an allowance for credit losses on its available-for-sale debt securities as of March 31, 2020. Fair Value of Financial Instruments The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. Accounting Standards Codification (ASC) 820, Fair Value Measurements and Disclosures • Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. • Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. • Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments. Net Loss Per Share Basic net loss per share is calculated by dividing net loss applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income loss per share until the contingency has been fully met. For purposes of the diluted net loss per share calculation, common stock equivalents are excluded from the calculation of diluted net loss per share if their effect would be anti-dilutive. Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740)—Simplifying the Accounting for Income Taxes

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2020
Investments Debt And Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following tables present a summary of the Company’s marketable securities, which consist of available-for-sale debt securities and equity securities (in thousands): Amortized Cost / Cost Unrealized Gains Unrealized Losses Fair Value March 31, 2020 U.S. government and federal agency securities $ 49,791 $ 378 $ — $ 50,169 Certificates of deposit 8,261 81 — 8,342 Corporate bonds 209,005 551 (730 ) 208,826 Equity securities 282 19,588 — 19,870 $ 267,339 $ 20,598 $ (730 ) $ 287,207 Amortized Cost / Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2019 U.S. government and federal agency securities $ 62,637 $ 215 $ (5 ) $ 62,847 Certificates of deposit 8,506 77 — 8,583 Corporate bonds 226,137 808 (29 ) 226,916 Equity securities 351 31,784 — 32,135 $ 297,631 $ 32,884 $ (34 ) $ 330,481 As of March 31, 2020 and December 31, 2019, no available-for-sale debt securities had remaining maturities greater than three years. The amortized cost of marketable debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, or to the earliest call date for callable debt securities purchased at a premium. As of March 31, 2020 and December 31, 2019, the balance in the Company’s accumulated other comprehensive income (loss) consisted solely of net unrealized gains and losses on available-for-sale debt securities, net of income tax effects and reclassification adjustments for realized gains and losses. During the three months ended March 31, 2020, the Company recognized net unrealized losses on available-for-sale debt securities of $0.8 million and income tax expense of zero in other comprehensive loss for the period. The Company recognized net realized gains of less than $0.1 million on the sale or maturity of available-for-sale debt securities during the three months ended March 31, 2020, which were reclassified out of accumulated other comprehensive income (loss) during the period and were included in investment income (loss) in the consolidated statements of operations and comprehensive loss. During the three months ended March 31, 2019, the Company recognized net unrealized gains on available-for-sale debt securities of $1.0 million and income tax expense of $0.4 million in other comprehensive income for the period. The Company recognized net realized gains of less than $0.1 million on the sale or maturity of available-for-sale debt securities during the three months ended March 31, 2019, which were reclassified out of accumulated other comprehensive income (loss) during the period and were included in investment income (loss) in the consolidated statements of operations and comprehensive loss. The following tables present the fair values and unrealized losses of available-for-sale debt securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses March 31, 2020 Corporate bonds $ 90,352 $ (730 ) $ — $ — $ 90,352 $ (730 ) $ 90,352 $ (730 ) $ — $ — $ 90,352 $ (730 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2019 U.S. government and federal agency securities $ 12,562 $ (5 ) $ — $ — $ 12,562 $ (5 ) Corporate bonds 48,556 (29 ) — — 48,556 (29 ) $ 61,118 $ (34 ) $ — $ — $ 61,118 $ (34 ) As of March 31, 2020, available-for-sale debt securities held by the Company which were in an unrealized loss position consisted of 30 investment grade security positions. The Company has the intent and ability to hold such securities until recovery, and due to the credit quality of the issuers and low severity of each unrealized loss position relative to its amortized cost basis, the Company has not identified any credit losses associated with its available-for-sale debt securities. The Company did not recognize any impairment or credit losses on available-for-sale debt securities during the three months ended March 31, 2020. Marketable equity securities held by the Company as of March 31, 2020 consisted solely of common stock of Prevail Therapeutics Inc. (Prevail). The Company acquired the securities as consideration for a commercial license to the NAV Technology Platform granted to Prevail in August 2017. Prevail completed its initial public offering (IPO) in June 2019. Prior to Prevail’s IPO, the securities were accounted for as non-marketable equity securities without a readily determinable fair value and had a carrying value of $0.4 million. Upon Prevail’s IPO in June 2019, the securities were reclassified to marketable securities and are measured at fair value. During the three months ended March 31, 2020, the Company recognized unrealized losses of $12.2 million and realized gains of $7.1 million related to its marketable equity securities of Prevail, which were included in investment income (loss) in the consolidated statements of operations and comprehensive loss.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	4. Fair Value of Financial Instruments Financial instruments reported at fair value on a recurring basis include cash equivalents and marketable securities. The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands): Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total March 31, 2020 Cash equivalents: Money market mutual funds $ — $ 54,530 $ — $ 54,530 Total cash equivalents — 54,530 — 54,530 Marketable securities: U.S. government and federal agency securities — 50,169 — 50,169 Certificates of deposit — 8,342 — 8,342 Corporate bonds — 208,826 — 208,826 Equity securities 19,870 — — 19,870 Total marketable securities 19,870 267,337 — 287,207 Total cash equivalents and marketable securities $ 19,870 $ 321,867 $ — $ 341,737 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2019 Cash equivalents: Money market mutual funds $ — $ 56,058 $ — $ 56,058 Total cash equivalents — 56,058 — 56,058 Marketable securities: U.S. government and federal agency securities — 62,847 — 62,847 Certificates of deposit — 8,583 — 8,583 Corporate bonds — 226,916 — 226,916 Equity securities 32,135 — — 32,135 Total marketable securities 32,135 298,346 — 330,481 Total cash equivalents and marketable securities $ 32,135 $ 354,404 $ — $ 386,539 Management estimates that the carrying amounts of its current accounts receivable, accounts payable and accrued expenses and other current liabilities approximate fair value due to the short-term nature of those instruments. Accounts receivable which contain non-current portions are recorded at their present values using a discount rate that is based on prevailing market rates and the credit profile of the licensee on the date the amounts are initially recorded. Management does not believe there have been any significant changes in market conditions or credit quality that would cause the discount rates initially used to be significantly different from those that would be used as of March 31, 2020 to determine the present value of the receivables. Accordingly, management estimates that the carrying value of its non-current accounts receivable approximates the fair value of those instruments. Non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. As of March 31, 2020 and December 31, 2019, the Company did not hold any non-marketable equity securities. No remeasurements or impairment losses were recorded on non-marketable equity securities during the three months ended March 31, 2020 and 2019.

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consists of the following (in thousands): March 31, 2020 December 31, 2019 Lab equipment $ 20,176 $ 19,663 Computer equipment and software 2,688 2,545 Furniture and fixtures 2,280 2,188 Leasehold improvements 21,565 18,915 Total property and equipment 46,709 43,311 Accumulated depreciation and amortization (16,295 ) (14,338 ) Property and equipment, net $ 30,414 $ 28,973

License and Royalty Revenue

License and Royalty Revenue	3 Months Ended
	Mar. 31, 2020
License Agreement Revenue Recognition [Abstract]
License and Royalty Revenue	6. License and Royalty Revenue As of March 31, 2020, the Company’s NAV Technology Platform was being applied by NAV Technology Licensees in one commercial product, Zolgensma, and in the development of more than 20 product candidates. Consideration to the Company under its license agreements may include: (i) up-front and annual fees, (ii) option fees to acquire additional licenses, (iii) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (iv) sublicense fees and (v) royalties on sales of licensed products. Sublicense fees vary by license and range from a mid-single digit percentage to a low-double digit percentage of license fees received by licensees as a result of sublicenses. Royalties on net sales of commercialized products vary by license and range from a mid-single digit percentage to a low double-digit percentage of net sales by licensees. Development milestone payments are evaluated each reporting period and are only included in the transaction price of each license and recognized as license revenue to the extent the milestones are considered probable of achievement. Sales-based milestones are excluded from the transaction price of each license agreement and recognized as royalty revenue in the period of achievement. As of March 31, 2020, the Company’s license agreements, excluding additional licenses that could be granted upon the exercise of options by licensees, contained unachieved milestones which could result in aggregate milestone payments to the Company of up to $371.4 million, including $0.3 million upon the submission of preclinical regulatory filings, $24.1 million upon the commencement of various stages of clinical trials, $31.0 million upon the submission of regulatory approval filings, $109.0 million upon the approval of commercial products by regulatory agencies and $207.0 million upon the achievement of specified sales targets for licensed products. To the extent the milestone payments are realized by the Company, the Company will be obligated to pay sublicense fees to licensors based on a specified percentage of the fees earned by the Company. The achievement of milestones by licensees is highly dependent on the successful development and commercialization of licensed products and it is at least reasonably possible that some or all of the milestone fees will not be realized by the Company. The following tables present changes in the balances of the Company’s receivables, contract assets and contract liabilities during the periods presented (in thousands): Balance at Beginning Balance at of Period Additions Deductions End of Period Three Months Ended March 31, 2020 Receivables and contract assets: Accounts receivable, current and non-current $ 42,303 $ 18,142 $ (11,550 ) $ 48,895 Contract assets $ — $ 350 $ — $ 350 Contract liabilities: Deferred revenue, current and non-current $ 3,333 $ — $ — $ 3,333 Balance at Beginning Balance at of Period Additions Deductions End of Period Three Months Ended March 31, 2019 Receivables and contract assets: Accounts receivable, current and non-current $ 31,599 $ 1,738 $ (2,207 ) $ 31,130 Contract assets $ 750 $ 1,000 $ (750 ) $ 1,000 Contract liabilities: Deferred revenue, current and non-current $ 3,933 $ — $ — $ 3,933 Additions to accounts receivable during the three months ended March 31, 2020 primarily consisted of royalties on net sales of Zolgensma of $10.0 million, receivables recorded related to new licenses granted during the period and interest income recognized during the period related to significant financing components. Additions to accounts receivable during the three months ended March 31, 2019 primarily consisted of amounts billed upon the achievement of development milestones by licensees and interest income recognized during the period related to significant financing components. Deductions to accounts receivable during the three months ended March 31, 2020 and 2019 primarily consisted of amounts collected from licensees during the period. The changes in the balances of contract assets during the three months ended March 31, 2020 and 2019 consist of development milestones deemed probable of achievement by licensees during the period, offset by the subsequent achievement of such milestones and billing of associated milestone payments by the Company. As of March 31, 2020, the Company had recorded deferred revenue of $3.3 million which represents consideration received from licensees for performance obligations that have not yet been satisfied by the Company. Unsatisfied performance obligations consist of options granted to licensees that provide material rights to the licensee to acquire additional licenses from the Company. These performance obligations will be satisfied, and underlying revenue will be recognized, upon the exercise or expiration of the options. The Company did not recognize any license revenue during the three months ended March 31, 2020 and 2019 that was included in deferred revenue at the beginning of the period. During the three months ended March 31, 2020 and 2019, the Company recognized revenue of $10.4 million and $0.8 million, respectively, from performance obligations satisfied in prior periods as a result of changes in the transaction prices of its license agreements as well as royalties on sales of licensed products and sublicense fees. Changes in the transaction prices during the periods were primarily attributable to development milestones achieved or deemed probable of achievement during the period that were previously not considered probable of achievement. As of March 31, 2020, the Company had recorded total current and non-current accounts receivable of $48.9 million, of which $7.1 million had been billed to customers and $41.8 million was billable to customers in future periods. As of December 31, 2019, the Company had recorded total current and non-current accounts receivable of $42.3 million, of which $0.4 million had been billed to customers and $41.9 million was billable to customers in future periods. Based on the Company’s evaluation of the credit quality and financial condition of its significant customers, history of collections and evaluation of current and future expected economic conditions, no credit losses were recognized on accounts receivable or contract assets during the three months ended March 31, 2020. AveXis March 2014 License In March 2014, the Company entered into an exclusive license agreement, as amended in January 2018 (the March 2014 License) with AveXis, Inc. (AveXis). Under the March 2014 License, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to the NAV Technology Platform, as well as other certain rights, for the treatment of spinal muscular atrophy (SMA) in humans by in vivo gene therapy. AveXis launched commercial sales of Zolgensma in the second quarter of 2019, which is a licensed product under the March 2014 License. Upon the commencement of commercial sales in the second quarter of 2019, the Company began recognizing royalty revenue on net sales of Zolgensma. The Company recognized the following amounts under the March 2014 License with AveXis (in thousands): Three Months Ended March 31, 2020 2019 License revenue $ — $ — Zolgensma royalty revenue 9,978 — Total license and royalty revenue $ 9,978 $ — Interest income from licensing $ 7 $ 8 As of March 31, 2020, the Company had recorded $10.0 million of accounts receivable from AveXis under the March 2014 License, of which $9.8 million were included in current assets and $0.2 million were included in non-current assets. As of December 31, 2019, the Company had recorded $11.0 million of accounts receivable from AveXis under the March 2014 License, of which $10.8 million were included in current assets and $0.2 million were included in non-current assets. Abeona Therapeutics Inc. Accounts receivable as of March 31, 2020 and December 31, 2019 included $26.9 million and $26.3 million, respectively, related to the November 2018 license agreement, as amended in November 2019 (the November 2018 License) with Abeona Therapeutics Inc. (Abeona) for the development and commercialization of treatments for various diseases, all of which were included in current assets. Pursuant to the November 2018 License, Abeona was required to pay the Company a license fee of $8.0 million on April 1, 2020. In May 2020, after providing written notice to Abeona as required under the license agreement, the November 2018 License was effectively terminated by the Company as a result of breach by Abeona for the failure to pay the $8.0 million license fee due on April 1, 2020. Pursuant to the November 2018 License, Abeona is required to pay the aforementioned $8.0 million license fee to the Company immediately upon the termination of the license agreement. Additionally, Abeona is required to pay a $20.0 million license fee to the Company within 15 days of the termination date, which otherwise would have been due to the Company in November 2020.

Stock-based Compensation

Stock-based Compensation	3 Months Ended
	Mar. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	7. Stock-based Compensation In January 2020, the Board of Directors authorized an additional 1,479,696 shares to be issued under the 2015 Equity Incentive Plan (the 2015 Plan). As of March 31, 2020, the total number of shares of common stock authorized for issuance under the 2015 Plan and the 2014 Stock Plan (the 2014 Plan) was 12,412,917, of which 2,454,658 remained available for future grants under the 2015 Plan. Stock-based Compensation Expense The Company’s stock-based compensation expense by award type was as follows (in thousands): Three Months Ended March 31, 2020 2019 Stock options $ 7,779 $ 5,452 Restricted stock units — 68 Employee stock purchase plan 238 198 $ 8,017 $ 5,718 As of March 31, 2020, the Company had $82.4 million of unrecognized stock-based compensation expense related to stock options and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.9 years. The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended March 31, 2020 2019 Research and development $ 4,047 $ 2,347 General and administrative 3,970 3,371 $ 8,017 $ 5,718 Stock Options The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data): Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2019 5,544 $ 28.79 7.5 $ 86,509 Granted 1,261 $ 39.12 Exercised (194 ) $ 11.64 Cancelled or forfeited (173 ) $ 44.35 Outstanding at March 31, 2020 6,438 $ 30.91 7.8 $ 52,998 Exercisable at March 31, 2020 3,202 $ 20.33 6.5 $ 49,710 Vested and expected to vest at March 31, 2020 6,438 $ 30.91 7.8 $ 52,998 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported. The weighted-average grant date fair value per share of options granted during the three months ended March 31, 2020 was $24.69. During the three months ended March 31, 2020, the total number of stock options exercised was 193,973, resulting in total proceeds of $2.3 million. The total intrinsic value of options exercised during the three months ended March 31, 2020 was $5.8 million. Employee Stock Purchase Plan In January 2020, the Board of Directors authorized an additional 369,924 shares to be issued under the 2015 ESPP. As of March 31, 2020, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 623,924, of which 486,068 remained available for future issuance. During the three months ended March 31, 2020, 17,442 shares of common stock were issued under the 2015 ESPP.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, including a three-year cumulative loss position as of March 31, 2020 and December 31, 2019, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for its net deferred tax assets as of March 31, 2020 and December 31, 2019. In response to the COVID-19 pandemic, the Coronavirus Aid, Relief and Economic Security Act (the CARES Act) was signed into law in March 2020. The CARES Act (i) lifts certain deduction limitations originally imposed by the Tax Cuts and Jobs Act of 2017 (the TCJA), (ii) allows corporate taxpayers to carryback net operating losses (NOLs) originating during 2018 through 2020 for up to five years, which was not previously allowed under the TCJA, (iii) eliminates the 80% of taxable income limitations on NOL utilization imposed by the TCJA, allowing corporate entities to fully utilize NOL carryforwards to offset taxable income in 2018, 2019 or 2020, and (iv) enacts various other changes to corporate taxation. The enactment of the CARES Act did not result in any material adjustments to the Company’s income tax provision for the three months ended March 31, 2020, or to the Company’s net deferred tax assets as of March 31, 2020.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	9. Related Party Transactions FOXKISER LLP Since 2016, the Company has been party to professional services agreements with FOXKISER LLP (FOXKISER), an affiliate of certain stockholders of the Company and an affiliate of a member of the Company’s Board of Directors, pursuant to which the Company pays a fixed monthly fee in consideration for certain strategic services provided by FOXKISER. Effective January 2019, the Company entered into a new professional services agreement with FOXKISER with similar terms and conditions as the previous agreements. The agreement was amended effective June 2019 to expand the scope of services provided and increase the monthly fee, and the amended agreement expires in December 2020. Expenses incurred under the agreements with FOXKISER for the three months ended March 31, 2020 and 2019 were $1.2 million and $0.8 million, respectively, and were recorded as research and development expenses in the consolidated statements of operations and comprehensive loss.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	10. Net Loss Per Share Since the Company incurred net losses for the three months ended March 31, 2020 and 2019, common stock equivalents were excluded from the calculation of diluted net loss per share as their effect would be anti-dilutive. Accordingly, basic and diluted net loss per share were the same for such periods. The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Three Months Ended March 31, 2020 2019 Stock options issued and outstanding 6,438 5,340 Unvested restricted stock units outstanding — 40 Employee stock purchase plan 32 20 6,470 5,400

Supplemental Disclosures

Supplemental Disclosures	3 Months Ended
	Mar. 31, 2020
Payables And Accruals [Abstract]
Supplemental Disclosures	11. Supplemental Disclosures Accrued expenses and other current liabilities consist of the following (in thousands): March 31, 2020 December 31, 2019 Accrued personnel costs $ 6,696 $ 10,903 Accrued sublicense fees and royalties 5,112 4,542 Accrued external research and development expenses 5,089 5,791 Accrued external general and administrative expenses 2,513 2,053 Accrued purchases of property and equipment 622 1,328 Other accrued expenses and current liabilities 295 229 $ 20,327 $ 24,846

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). The interim unaudited consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements as of and for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, which was filed with the SEC on February 26, 2020. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which are normal and recurring in nature, necessary for fair financial statement presentation. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year, any other interim periods, or any future year or period. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements. Significant estimates are used in the following areas, among others: license and royalty revenue, stock-based compensation expense, accrued research and development expenses and other accrued liabilities, income taxes and the fair value of financial instruments. The Company is actively monitoring the impact of the novel coronavirus (COVID-19) pandemic on its business, results of operations and financial condition. The full extent to which COVID-19 will directly or indirectly impact the Company’s business, results of operations and financial condition in the future is unknown at this time and will depend on future developments that are highly unpredictable. The most significant estimates affecting the Company’s consolidated financial statements that may be impacted by the COVID-19 pandemic are related to the Company’s assessment of credit losses on accounts receivable, contract assets and available-for-sale debt securities.
Reclassifications	Reclassifications Certain amounts reported in prior periods have been reclassified to conform to current period financial statement presentation. These reclassifications are not material and have no effect on previously reported financial position, results of operations and cash flows.
Restricted Cash	Restricted Cash Restricted cash includes money market mutual funds used to collateralize irrevocable letters of credit as required by the Company’s lease agreements. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): March 31, 2020 March 31, 2019 Cash and cash equivalents $ 69,411 $ 55,852 Restricted cash 1,330 1,053 Total cash and cash equivalents and restricted cash $ 70,741 $ 56,905
Accounts Receivable	Accounts Receivable Accounts receivable primarily consist of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable include amounts invoiced to licensees as well as rights to consideration which have not yet been invoiced, including unbilled royalties, and for which payment is conditional solely upon the passage of time. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any accounts receivable from the licensee which are not contractually payable to the Company are charged off as a reduction of license revenue in the period of the termination. Accounts receivable which are not expected to be received by the Company within 12 months from the reporting date are stated net of a discount to present value and recorded as non-current assets on the consolidated balance sheets. The present value discount is recognized as a reduction of revenue in the period in which the accounts receivable are initially recorded and is accreted as interest income from licensing over the term of the receivables. Accounts receivable are stated net of an allowance for doubtful accounts, if deemed necessary based on the Company’s evaluation of collectability and potential credit losses. Management assesses the collectability of its accounts receivable using the specific identification of account balances, and considers the credit quality and financial condition of its significant customers, historical information regarding credit losses and the Company’s evaluation of current and expected future economic conditions. If necessary, an allowance for doubtful accounts is recorded against accounts receivable such that the carrying value of accounts receivable reflects the net amount expected to be collected. The Company did no t record an allowance for doubtful accounts as of March 31, 2020 and December 31, 2019 .
Marketable Securities	Marketable Securities Marketable securities consist of available-for-sale debt securities and equity securities and are carried at fair value. Marketable debt securities with remaining maturity dates exceeding 12 months which are not intended to be sold prior to maturity for use in current operations are classified as non-current assets. Marketable equity securities are classified as current assets. Unrealized gains and losses on available-for-sale debt securities, net of any related tax effects, are excluded from results of operations and are included in other comprehensive income (loss) and reported as a separate component of stockholders’ equity until realized. The Company uses the aggregate portfolio approach to release the tax effects of unrealized gains and losses on available-for-sale debt securities in accumulated other comprehensive income (loss). Purchase premiums and discounts on marketable debt securities are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Unrealized gains and losses on marketable equity securities are included in results of operations as investment income (loss). Realized gains and losses from the sale or maturity of marketable securities are based on the specific identification method and are included in results of operations as investment income (loss). At each reporting date, the Company evaluates available-for-sale debt securities which have an amortized cost basis in excess of the fair value of the security to determine if the unrealized loss or any potential credit losses should be recognized in results of operations . If the Company does not have the intent and ability to hold the security until recovery of the unrealized loss, the difference between the fair value and amortized cost basis of the security is charged to results of operations resulting in a new amortized cost basis of the security. If the Company has the intent and ability to hold the security until recovery of the unrealized loss, the security is evaluated for potential credit losses. If a credit loss is deemed to exist, the credit loss is recognized in results of operations and an allowance for credit losses is recorded against the amortized cost basis of the security. In determining whether a credit loss exists related to impaired available-for-sale debt securities, the Company considers, among other factors, the extent of the unrealized loss relative to the amortized cost basis, the credit rating of the issuer and any recent changes thereto, current and expected future economic conditions, and any adverse events or other changes in circumstances that have occurred which may indicate a potential credit loss. The Company did not record an allowance for credit losses on its available-for-sale debt securities as of March 31, 2020.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. Accounting Standards Codification (ASC) 820, Fair Value Measurements and Disclosures • Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. • Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. • Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing net loss applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income loss per share until the contingency has been fully met. For purposes of the diluted net loss per share calculation, common stock equivalents are excluded from the calculation of diluted net loss per share if their effect would be anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740)—Simplifying the Accounting for Income Taxes

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): March 31, 2020 March 31, 2019 Cash and cash equivalents $ 69,411 $ 55,852 Restricted cash 1,330 1,053 Total cash and cash equivalents and restricted cash $ 70,741 $ 56,905

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2020
Investments Debt And Equity Securities [Abstract]
Summary of Company Marketable Securities	The following tables present a summary of the Company’s marketable securities, which consist of available-for-sale debt securities and equity securities (in thousands): Amortized Cost / Cost Unrealized Gains Unrealized Losses Fair Value March 31, 2020 U.S. government and federal agency securities $ 49,791 $ 378 $ — $ 50,169 Certificates of deposit 8,261 81 — 8,342 Corporate bonds 209,005 551 (730 ) 208,826 Equity securities 282 19,588 — 19,870 $ 267,339 $ 20,598 $ (730 ) $ 287,207 Amortized Cost / Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2019 U.S. government and federal agency securities $ 62,637 $ 215 $ (5 ) $ 62,847 Certificates of deposit 8,506 77 — 8,583 Corporate bonds 226,137 808 (29 ) 226,916 Equity securities 351 31,784 — 32,135 $ 297,631 $ 32,884 $ (34 ) $ 330,481
Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater	The following tables present the fair values and unrealized losses of available-for-sale debt securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses March 31, 2020 Corporate bonds $ 90,352 $ (730 ) $ — $ — $ 90,352 $ (730 ) $ 90,352 $ (730 ) $ — $ — $ 90,352 $ (730 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2019 U.S. government and federal agency securities $ 12,562 $ (5 ) $ — $ — $ 12,562 $ (5 ) Corporate bonds 48,556 (29 ) — — 48,556 (29 ) $ 61,118 $ (34 ) $ — $ — $ 61,118 $ (34 )

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Cash Equivalents and Marketable Securities	The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands): Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total March 31, 2020 Cash equivalents: Money market mutual funds $ — $ 54,530 $ — $ 54,530 Total cash equivalents — 54,530 — 54,530 Marketable securities: U.S. government and federal agency securities — 50,169 — 50,169 Certificates of deposit — 8,342 — 8,342 Corporate bonds — 208,826 — 208,826 Equity securities 19,870 — — 19,870 Total marketable securities 19,870 267,337 — 287,207 Total cash equivalents and marketable securities $ 19,870 $ 321,867 $ — $ 341,737 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2019 Cash equivalents: Money market mutual funds $ — $ 56,058 $ — $ 56,058 Total cash equivalents — 56,058 — 56,058 Marketable securities: U.S. government and federal agency securities — 62,847 — 62,847 Certificates of deposit — 8,583 — 8,583 Corporate bonds — 226,916 — 226,916 Equity securities 32,135 — — 32,135 Total marketable securities 32,135 298,346 — 330,481 Total cash equivalents and marketable securities $ 32,135 $ 354,404 $ — $ 386,539

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net consists of the following (in thousands): March 31, 2020 December 31, 2019 Lab equipment $ 20,176 $ 19,663 Computer equipment and software 2,688 2,545 Furniture and fixtures 2,280 2,188 Leasehold improvements 21,565 18,915 Total property and equipment 46,709 43,311 Accumulated depreciation and amortization (16,295 ) (14,338 ) Property and equipment, net $ 30,414 $ 28,973

License and Royalty Revenue (Ta

License and Royalty Revenue (Tables)	3 Months Ended
	Mar. 31, 2020
License Agreement Revenue Recognition [Abstract]
Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities	The following tables present changes in the balances of the Company’s receivables, contract assets and contract liabilities during the periods presented (in thousands): Balance at Beginning Balance at of Period Additions Deductions End of Period Three Months Ended March 31, 2020 Receivables and contract assets: Accounts receivable, current and non-current $ 42,303 $ 18,142 $ (11,550 ) $ 48,895 Contract assets $ — $ 350 $ — $ 350 Contract liabilities: Deferred revenue, current and non-current $ 3,333 $ — $ — $ 3,333 Balance at Beginning Balance at of Period Additions Deductions End of Period Three Months Ended March 31, 2019 Receivables and contract assets: Accounts receivable, current and non-current $ 31,599 $ 1,738 $ (2,207 ) $ 31,130 Contract assets $ 750 $ 1,000 $ (750 ) $ 1,000 Contract liabilities: Deferred revenue, current and non-current $ 3,933 $ — $ — $ 3,933
Schedule of License Revenue	The Company recognized the following amounts under the March 2014 License with AveXis (in thousands): Three Months Ended March 31, 2020 2019 License revenue $ — $ — Zolgensma royalty revenue 9,978 — Total license and royalty revenue $ 9,978 $ — Interest income from licensing $ 7 $ 8

Stock-based Compensation (Table

Stock-based Compensation (Tables)	3 Months Ended
	Mar. 31, 2020
Stock-Based Compensation Expense by Award Type	The Company’s stock-based compensation expense by award type was as follows (in thousands): Three Months Ended March 31, 2020 2019 Stock options $ 7,779 $ 5,452 Restricted stock units — 68 Employee stock purchase plan 238 198 $ 8,017 $ 5,718
Stock-Based Compensation Expense	The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended March 31, 2020 2019 Research and development $ 4,047 $ 2,347 General and administrative 3,970 3,371 $ 8,017 $ 5,718
2014 and 2015 Equity Incentive Plan [Member]
Summary of Stock Option Activity	The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data): Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2019 5,544 $ 28.79 7.5 $ 86,509 Granted 1,261 $ 39.12 Exercised (194 ) $ 11.64 Cancelled or forfeited (173 ) $ 44.35 Outstanding at March 31, 2020 6,438 $ 30.91 7.8 $ 52,998 Exercisable at March 31, 2020 3,202 $ 20.33 6.5 $ 49,710 Vested and expected to vest at March 31, 2020 6,438 $ 30.91 7.8 $ 52,998 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Schedules for Computation of Diluted Weighted-Average Shares Outstanding	The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Three Months Ended March 31, 2020 2019 Stock options issued and outstanding 6,438 5,340 Unvested restricted stock units outstanding — 40 Employee stock purchase plan 32 20 6,470 5,400

Supplemental Disclosures (Table

Supplemental Disclosures (Tables)	3 Months Ended
	Mar. 31, 2020
Payables And Accruals [Abstract]
Schedules of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following (in thousands): March 31, 2020 December 31, 2019 Accrued personnel costs $ 6,696 $ 10,903 Accrued sublicense fees and royalties 5,112 4,542 Accrued external research and development expenses 5,089 5,791 Accrued external general and administrative expenses 2,513 2,053 Accrued purchases of property and equipment 622 1,328 Other accrued expenses and current liabilities 295 229 $ 20,327 $ 24,846

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (217,860)	$ (177,822)
Cash, cash equivalents and marketable securities	$ 356,600

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Accounting Policies [Abstract]
Cash and cash equivalents	$ 69,411	$ 69,514	$ 55,852
Restricted cash	1,330	$ 1,330	1,053
Total cash and cash equivalents and restricted cash	$ 70,741		$ 56,905

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Allowance for doubtful accounts receivable	$ 0	$ 0
Allowance for credit losses on available-for-sale debt securities	$ 0

Marketable Securities - Summary

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	$ 267,339	$ 297,631
Unrealized Gains	20,598	32,884
Unrealized Losses	(730)	(34)
Fair Value	287,207	330,481
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	49,791	62,637
Unrealized Gains	378	215
Unrealized Losses		(5)
Fair Value	50,169	62,847
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	8,261	8,506
Unrealized Gains	81	77
Fair Value	8,342	8,583
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	209,005	226,137
Unrealized Gains	551	808
Unrealized Losses	(730)	(29)
Fair Value	208,826	226,916
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	282	351
Unrealized Gains	19,588	31,784
Fair Value	$ 19,870	$ 32,135

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020USD ($)Security	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale debt securities remaining maturities greater than three years	$ 0		$ 0
Unrealized gains (losses) on available-for-sale debt securities, before tax	$ (800,000)	$ 1,000,000
Number of investment grade fixed income debt security | Security	30
Other-than-temporary impaired	$ 0
Non-marketable equity securities, carrying value	400,000
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, unrealized gain (loss)	(12,200,000)
Marketable securities, realized gain	7,100,000
Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Unrealized gains (losses) on available-for-sale debt securities, income tax expense	0	400,000
Investment Income [Member] | Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains (losses) available for sale securities	$ 100,000	$ 100,000

Marketable Securities - Summa_2

Marketable Securities - Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 90,352
Less than 12 Months, Unrealized Losses	(730)	$ (34)
Total, Fair Value	90,352
Total, Unrealized Losses	(730)	(34)
Less than 12 Months, Fair Value		61,118
Total, Fair Value		61,118
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	90,352
Less than 12 Months, Unrealized Losses	(730)	(29)
Total, Fair Value	90,352
Total, Unrealized Losses	$ (730)	(29)
Less than 12 Months, Fair Value		48,556
Total, Fair Value		48,556
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Unrealized Losses		(5)
Total, Unrealized Losses		(5)
Less than 12 Months, Fair Value		12,562
Total, Fair Value		$ 12,562

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Marketable securities:
Fair Value	$ 287,207	$ 330,481
Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	341,737	386,539
Corporate Bonds [Member]
Marketable securities:
Fair Value	208,826	226,916
Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	54,530	56,058
U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	50,169	62,847
Certificates of Deposit [Member]
Marketable securities:
Fair Value	8,342	8,583
Equity Securities [Member]
Marketable securities:
Fair Value	19,870	32,135
Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	54,530	56,058
Quoted Prices in Active Markets (Level 1) [Member]
Marketable securities:
Fair Value	19,870	32,135
Quoted Prices in Active Markets (Level 1) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	19,870	32,135
Quoted Prices in Active Markets (Level 1) [Member] | Equity Securities [Member]
Marketable securities:
Fair Value	19,870	32,135
Significant Other Observable Inputs (Level 2) [Member]
Marketable securities:
Fair Value	267,337	298,346
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	321,867	354,404
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Marketable securities:
Fair Value	208,826	226,916
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	54,530	56,058
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	50,169	62,847
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Marketable securities:
Fair Value	8,342	8,583
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	$ 54,530	$ 56,058

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Non-marketable equity securities	$ 0		$ 0
Remeasurements or impairment losses on non-marketable equity securities	$ 0	$ 0

Property and Equipment Net - Sc

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property and equipment	$ 46,709	$ 43,311
Accumulated depreciation and amortization	(16,295)	(14,338)
Property and equipment, net	30,414	28,973
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	20,176	19,663
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	2,688	2,545
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	2,280	2,188
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 21,565	$ 18,915

License and Royalty Revenue - A

License and Royalty Revenue - Additional Information (Detail)	Apr. 01, 2020USD ($)	Mar. 31, 2020USD ($)ProductCandidate	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
License Revenue [Line Items]
Milestone fee upon submission of preclinical regulatory filings		$ 300,000
Milestone payment upon commencement of clinical trials in humans		24,100,000
Milestone payment upon submission of regulatory approval filings		31,000,000
Milestone payment upon approval of commercial products by regulatory agencies		109,000,000
Milestone payment upon achievement of specified sales targets for licensed products		207,000,000
Accounts receivable		48,895,000	$ 31,130,000	$ 42,303,000	$ 31,599,000
Deferred revenue, current and non-current		3,333,000	3,933,000	3,333,000	3,933,000
Licenses revenue		17,644,000	884,000
Credit losses on receivables or contract assets		0
Total current and non-current accounts receivable		48,895,000	31,130,000	42,303,000	$ 31,599,000
Billed to customers		7,100,000		400,000
Billable to customers in future periods		41,800,000		41,900,000
Accounts receivable, current		44,522,000		38,148,000
Abeona Therapeutics Inc. [Member] | November 2018 License Agreement Termination [Member] | Subsequent Event
License Revenue [Line Items]
Up-front and anuual fees	$ 20,000,000
March 2014 License Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Accounts receivable		10,000,000		11,000,000
Accounts receivable, current		9,800,000		10,800,000
Accounts receivable, non -current		200,000		200,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Accounts receivable		26,900,000		$ 26,300,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member] | Subsequent Event
License Revenue [Line Items]
License fee	8,000,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member] | November 2018 License Agreement Termination [Member] | Subsequent Event
License Revenue [Line Items]
License fee	$ 8,000,000
License agreement termination claims, description	Additionally, Abeona is required to pay a $20.0 million license fee to the Company within 15 days of the termination date, which otherwise would have been due to the Company in November 2020.
Maximum [Member]
License Revenue [Line Items]
Aggregate milestone payment for all the targets		$ 371,400,000
N A V Technology Platform [Member]
License Revenue [Line Items]
Number of commercial product candidates | ProductCandidate		1
N A V Technology Platform [Member] | Minimum [Member]
License Revenue [Line Items]
Number of development partnered product candidates | ProductCandidate		20
Zolgensma Royalties [Member]
License Revenue [Line Items]
Accounts receivable		$ 10,000,000
Total current and non-current accounts receivable		10,000,000
License [Member]
License Revenue [Line Items]
Licenses revenue		0
License [Member] | License Agreement [Member]
License Revenue [Line Items]
License revenue		$ 10,400,000	$ 800,000

License and Royalty Revenue - S

License and Royalty Revenue - Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Receivables and contract assets:
Accounts receivable, current and non-current, Balance at Beginning of Period	$ 42,303	$ 31,599
Additions Related To Accounts Receivable	18,142	1,738
Deductions Related To Accounts Receivable	(11,550)	(2,207)
Accounts receivable, current and non-current, Balance at End of Period	48,895	31,130
Contract assets, Balance at Beginning of Period		750
Additions Related To Contract With Customer Assets	350	1,000
Deductions Related To Contract With Customer Asset		(750)
Contract assets, Balance at End of Period	350	1,000
Contract liabilities:
Deferred revenue, current and non-current, Balance at Beginning of Period	3,333	3,933
Deferred revenue, current and non-current, Balance at End of Period	$ 3,333	$ 3,933

License and Royalty Revenue -_2

License and Royalty Revenue - Schedule Of License Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
License Revenue [Line Items]
Total license and royalty revenue	$ 9,978
Interest income from licensing	848	$ 613
AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing	7	$ 8
Zolgensma Royalties [Member]
License Revenue [Line Items]
Total license and royalty revenue	$ 9,978

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2020	Mar. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 82.4
Unrecognized stock-based compensation, weighted-average period		2 years 10 months 24 days
Proceeds from stock options exercised including unsettled options		$ 2.3
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,479,696
Common stock shares authorized for issuance		12,412,917
Shares available for future grants		2,454,658
Weighted-average fair values of options granted		$ 24.69
Exercise of stock options, Shares		193,973
Total intrinsic value of options exercised		$ 5.8
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	369,924
Common stock shares authorized for issuance		623,924
Shares available for future grants		486,068
Common stock shares issued to participants		17,442

Stock-based Compensation - Stoc

Stock-based Compensation - Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 8,017	$ 5,718
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	7,779	5,452
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		68
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 238	$ 198

Stock-based Compensation - St_2

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 8,017	$ 5,718
Research and Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	4,047	2,347
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,970	$ 3,371

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock Option Activity (Detail) - 2014 and 2015 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,544
Shares, Granted	1,261
Shares, Exercised	(194)
Shares, Cancelled or forfeited	(173)
Shares Outstanding, Ending Balance	6,438	5,544
Shares, Exercisable	3,202
Shares, Vested and expected to vest	6,438
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 28.79
Weighted-average Exercise Price, Granted	39.12
Weighted-average Exercise Price, Exercised	11.64
Weighted-average Exercise Price, Cancelled or forfeited	44.35
Weighted-average Exercise Price, Outstanding, Ending Balance	30.91	$ 28.79
Weighted-average Exercise Price, Exercisable	20.33
Weighted-average Exercise Price, Vested and expected to vest	$ 30.91
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	7 years 9 months 18 days	7 years 6 months
Weighted-average Remaining Contractual Life (Years), Exercisable	6 years 6 months
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	7 years 9 months 18 days
Aggregate Intrinsic Value Outstanding	$ 52,998	$ 86,509
Aggregate Intrinsic Value, Exercisable	49,710
Aggregate Intrinsic Value, Vested and expected to vest	$ 52,998

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail)	1 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Elimination of limitation in net operating losses deductions as percentage of current year taxable income, percentage	80.00%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
FOXKISER LLP [Member] | Service Agreements [Member] | Research and Development [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 1.2	$ 0.8

Net Loss Per Share - Schedules

Net Loss Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	6,470	5,400
Stock Options Issued and Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	6,438	5,340
Unvested Restricted Stock Units Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding		40
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	32	20

Supplemental Disclosures - Sche

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued personnel costs	$ 6,696	$ 10,903
Accrued sublicense fees and royalties	5,112	4,542
Accrued external research and development expenses	5,089	5,791
Accrued external general and administrative expenses	2,513	2,053
Accrued purchases of property and equipment	622	1,328
Other accrued expenses and current liabilities	295	229
Accrued expenses and other current liabilities	$ 20,327	$ 24,846